Employee pay...by the hr or piece work?

I have a new helper. He is really quite slow. He's been working for me about a month and we are now almost 2.5 hrs longer than I was with my old helper. I was wondering if I payed him by the yard ($4.50 or $5 per yard) if that would give him incentive to speed up a bit. Our yards used to average about 15-20 minutes but now are pushing 25-30 minutes. He could make better money on the by yard price if he picked up the pace. When I was working for a guy during my university, I got $8 per lawn. Some yards you were making $30 an hour, some you were making $10 (just do to the size) but I averaged about $14.

What do you guys think? Do any of you go by the job for employee pay? Thanks for the input.

Grassroots,
Have you discussed his "slowness"? I ask because anyone can let someone go. It takes a sharp person to turn someone around. Some people don't know they're stats, responsiblities, or standing.